Post-effects of nandrolone decanoate treatment on contractile responses of rat skeletal muscles.

The post-effects of nandrolone decanoate treatment (15 mg.kg(-1).week(-1)) were studied on contractile responses of isolated small bundles of intact slow-(soleus) and fast-(extensor digitorum longus, edl) twitch fibers in rat. Five weeks of treatment induced, in edl, an increase in the amplitude of twitch (55%) and K(+) contracture (32%) without significant change in the time constant of relaxation and caffeine contracture. In soleus, an increase in the amplitude of twitch (35%) and caffeine contracture (0.2 mM: 218%, 0.5 mM: 88%, 5 mM: 28%, and 10 mM: 25%) was observed without change in K(+) contracture characteristics. In addition to these effects, 10 weeks of drug treatment increased the amplitude of soleus K(+) contracture (29%) and edl caffeine contracture (0.2 mM: 247%, 0.5 mM: 170%, 5 mM: 29%, and 10 mM: 45%), and reduced the 50% recovery time for K(+) contracture (EC(50)) (soleus: 37%, edl: 12%). After 5-week treatment followed by 5-week arrest, as compared to the control group, no change in the amplitude of twitch, K(+) or caffeine contractures was found in edl. In soleus, although no significant difference was observed in the amplitude of twitch, the amplitude of K(+) (35%) and caffeine contractures (0.2 mM: 227%, 0.5 mM: 128%) remained greater than the control group. Moreover, the EC(50) values were prolonged (46%) in soleus, whereas no significant difference was observed in edl. The present work suggests that nandrolone decanoate treatment induced differential post-effects on contractile responses developed in slow- and fast-twitch skeletal muscles by acting differently on the different steps of the excitation-contraction coupling mechanism.